A diffuser converts kinetic energy of fluid exiting an impeller into static pressure. The diffuser ideally provides good pressure recovery over the full range of flow angles under which the compressor operates. Vaned diffusers provide excellent pressure recovery but over a limited operating range only. Vaneless diffusers, on the other hand, have a broad operating range but provide only modest pressure recovery.
Certain appliances may experience a wide range of loads and flow rates. For example, a vacuum cleaner may experience flow rates of between 5 and 35 l/s as the cleaner is manoeuvred over different floor surfaces. Generally, however, the motor speed for these appliances is relatively slow, typically below 50 krpm. At these relatively low speeds, changes in flow rate effect only modest changes in the flow angle. Consequently, even though the appliance experiences a wide range of flow rates, the operating range is relatively small. However, as advances in motor technology lead to smaller and faster motors, the influence of flow rate on flow angle becomes an increasing problem. At speeds of around 100 krpm, even a modest change in flow rate can effect a relatively large change in flow angle. There is therefore a growing need for diffusers that can provide good pressure recovery over a relatively broad operating range.
Variable-geometry diffusers employ vanes having a stagger angle that varies with flow angle. By varying the geometry of the vanes in response to changes in flow, the diffuser can provide good pressure over a broad operating range. However, variable-geometry diffusers are expensive, require complex control, and are more prone to failure due to the presence of moving parts.